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To acclimatize to extreme altitude oxygen levels, why do Everest (Chomolungma) climbers go first up and then back down before going up again, instead of staying at say Camp II (21,000 ft; 6,400 m) for several days longer before continuing climbing to Camp III (23,500 ft; 7150 m)? Is acclimatizing by the other pattern faster and if so, why?

Of course, on Camp IV (South Col) at 26,000 ft (7900 m) permanent acclimatization is impossible, and one cannot stay there permanently, my question is concerning the lower camps (including Base Camp).

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Basically what happens is that the body needs some time to start producing more red blood cells. If you stay at the higher level your body can't cope with the acute lack of oxygen and as a result you run the risk of altitude sickness, pulmonary edema, cerebral edema and death*. Altitude sickness can occur at heights above about 2500 m, so you don't even need to be all that high before the symptoms show.

The easiest way to prevent altitude sickness is to return to a lower level, preferably below the 2500 m level, but a level to which you are already acclimatized may suffice. Doing so gives the body some time to start making more red blood cells in response to the lower level of oxygen experienced at the high altitude. When you return to the higher altitude, your body has now adapted and you can function better. This process is often repeated several times, increasing the time spent at the higher altitude with each visit, so that the body can fully adapt.

Acclimatization processes are not a guarantee that you won't still get altitude sickness at the higher level, they just make it less likely. As you know already, there is a level above which people can not adapt, no matter how many times they go up and down, this is known as the death zone.

* what? you don't know who death is?

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    But can't you simply stay longer at the lower altitudes to adapt? Instead of going above 2.5 km and then down before going up again, why didn't you spend one or more nights just below 2.5 km to adapt? It's an altitude you'd be again on, after all, so your body wouldn't need to adapt to even higher altitudes if it's lower again, would it? Is the acclimatization faster then or don't we just know if the body would handle it in the first place?
    – Giovanni
    Oct 18, 2021 at 6:26
  • @Giovanni yes, the acclimatization is faster when you stay a few hours in the zone above your current climatization, I have no scientific data, but it's exactly the reason all expeditions are doing that. Oct 18, 2021 at 6:44
  • @Giovanni - that's exactly what happens. The problem is that your body doesn't adapt to altitudes higher than the one you go to. However, there's a whole field of research etc into high-altitude performance that you can peruse for yourself if you are interested in mechanisms and what goes on. A lot of athletes do this sort of thing to prepare for major competitions. It's called altitude training and apparently can be mimicked by breathing through a narrow diameter tube to limit inspiration during exercise.
    – bob1
    Oct 18, 2021 at 7:04
  • @Giovanni the rule of thumb is "work high sleep low". When you exert yourself at a higher altitude, your body is forced to produce more RBC. But you don't immediately produce enough RBC to actually allow you to safely camp at that higher altitude. So it is safer to come down to sleep. When you do this cycle a few times, the body is supposed to have gotten sufficiently acclimatized to the higher altitude to allow you to camp there. This is also why there are guidelines on how much altitude to gain in one day.
    – ahron
    Nov 12, 2021 at 16:29
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Acclimatization is like training a muscle. You create a stimulus and recover. If you stay at a high altitude, your recovery will be limited and acclimatization will be slow.

Note that while this is the main reason, there is a whole array of practical points as well:

  • Advancing from camp to camp (without descending) will severely limit the altitude difference between camps. There is probably not enough suitable camp sites on the mountain. Literature suggests a maximum of 300-600 vertical meters between camps and a rest day every 2-3 days
  • Logistics: The equipment has to be carried up to higher camps. The less time people spend in higher camps, the less food they need. Also the level of comfort can be lower if one is just spending a night as opposed to a week, saving weight on tents, etc
  • Weather: Basically the same point as logistics. By moving fast and staying on the mountain only for a short time, the risk of adverse weather is limited.

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